Reset page allocator on new layout, only set mask regions for root layer.
Reset page allocator on new layout, only set mask regions for root layer.

These are some perf fixes for the transparent rects demo. There are still some perf regressions that remain, separate fixes will be needed for those.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,alanamramjit/webrender,alanamramjit/webrender,servo/webrender,servo/webrender